Bonding pads are interfaces between integrated circuits contained in semiconductor chips and a device package. Each bonding pad is connected to one or more contact pads on an IC-mounting surface of the device package through wire bonding, tape automated bonding, or flip chip technologies. Copper exhibits higher conductivity and greater mechanical integrity, and therefore becomes the material of choice for interconnection wires, contacts and bond pads in future generations. An insulating material that possesses a dielectric constant as low as possible, such as a dielectric constant (k) below the dielectric constant of about 3.9 of silicon dioxide, has been used in copper back-end-of-line (BEOL) interconnection technologies to avoid increased problems of capacitive coupling (cross-talk) and propagation delay. Copper is essentially a material that is quite easily oxidized and easily enhances its oxidation due to moisture in air, thus an aluminum pad providing high resistance against oxidation is used to cap an upper portion of copper interconnection. This aluminum pad allows use of the same bonding tools and processes used in aluminum interconnection technologies. When the aluminum pad is deposited on the copper layer, however, most of aluminum is consumed reacting with the underlying copper, forming CuAl2. A diffusion barrier layer of tantalum nitride is necessary, inserted between the aluminum pad and the underlying copper layer, to prevent the reaction between these metals. U.S. Pat. No. 6,350,667 incorporated herein by reference, describes an adhesion aluminum layer inserted between tantalum nitride and copper for improving adhesion in the pad metal stack structure.
Modern IC designs with high circuit density require a significantly increased number of pins and bonding pads to reduce bonding pad pitch and size. Large mechanical stresses inherent in bonding operations, however, easily damage smaller bonding pads. When an IC chip is under a wafer accept test (WAT), such as a wafer probing test or the like, a probe pin may damage the soft surface of the aluminum pad, and thereby the underlying copper layer is exposed to air and may be corroded. The corroded pads caused by this type of pad voids degrade the bondability of wire connection. In package processing, including wafer sorting, wire bonding, flip-chip packaging, or probe pin testing, applied forces or large mechanical stresses may crack an inter-metal dielectric (IMD) layer adjacent to a probe pin region. The crack may extend into the interior of the IMD layer surrounding the top level copper layer, causing corrosion and layer-open problems. This also causes the aluminum pad to peel from the top level copper layer, thus the pad-open problem causes the wire to lose contact with the aluminum pad, decreasing bonding reliability. Additionally, the pitch and size of the bonding pad cannot be further shrunk as the bonding pad is susceptible to damage from the mechanical stress, thus limiting chip size reduction in next generation technologies.